Recently, the design rules of semiconductor elements forming LSIs are still more miniaturized in order to meet demands for a high integration degree and high speed of LSIs. Accordingly, a demand has arisen for decreasing the resistance of the gate electrode of a transistor used in a DRAM or flash memory. Although polysilicon is conventionally used as the gate electrode, polysilicon has the drawback that the sheet resistance is high. Therefore, as a metal having a low resistance value, high adhesion to a silicon oxide film and silicon itself, and high processability, it has been proposed to stack a refractory metal such as tungsten or its silicide on a polysilicon layer. More specifically, tungsten polycide (a stacked film of WSi/poly-Si) and a tungsten poly-metal gate (a stacked structure of W/WN/poly-Si) are attracting attention. Note that WN of the tungsten poly-metal gate is a barrier layer (diffusion barrier layer) for preventing the reaction between tungsten and polysilicon.
The gate of a transistor is generally formed in the order of a well, gate insulating film, and gate electrode. Etching is performed to form the gate electrode. Since this etching exposes the side surfaces of a polysilicon layer in the gate electrode, electric field concentration occurs in this exposed region when a voltage is applied to the gate electrode, and this causes a product defect such as an increase in leakage current. Therefore, it is necessary to perform an oxidation process of forming insulating films by oxidizing the exposed regions of polysilicon in the gate electrode.
As a method of the oxidation process of forming insulating films on the exposed regions of the polysilicon layer on the side surfaces of the gate electrode, a method that performs thermal oxidation at a high temperature of 800° C. or more is generally used (e.g., Patent Document 1). When thermal oxidation is performed, however, a phenomenon (bird's beak) in which the edges of the polysilicon layer excessively oxidize occurs, and this makes the film thickness of the gate insulating film less uniform in the direction of the plane, and makes it difficult to meet the ultra-miniaturization design rules.
Also, tungsten and tungsten silicide used to decrease the sheet resistance of the gate electrode are rapidly oxidized if the temperatures exceed about 300° C. and about 400° C., respectively, and this poses the problem that a metal layer is oxidized if thermal oxidation is performed at a high temperature exceeding 800° C.
[Patent Document 1]
Jpn. Pat. Appln. KOKAI Publication No. 2000-269496 (the scope of claims and the like)